|An ion is an atom with a charge. There are two kinds of charges, positive and negative and the electronic structures of the elements affect the kinds of ions that they form.|
|We will start with the simplest of all atoms, hydrogen. An
of hydrogen has one proton in the nucleus and one electron in
Since protons are positive and electrons are negative then the charges
on the proton and electron cancel out. An atom of hydrogen is
neutral. If the hydrogen loses its electron (the reason why will be
for later) then we are left with just a proton in the nucleus.
is symbolized by H+. This is now a hydrogen ion.
It has a charge and a much smaller radius. In chemistry we write
H0 ----> H+ + e- "neutral hydrogen --> a hydrogen ion and an electron"
This can also be written as: H0
H+ + e-
|For a second example we will look at sodium, Na0.
A neutral sodium atom has 11 protons and 11 electrons. It is
because the 11 positive protons exactly cancel out the 11 negative
If sodium loses an electron the sodium becomes an ion with a positive 1 charge. This is because the 11 positive protons and 10 negative electrons end up with an overall charge of +1.
11 protons + 10 electrons = +1
+11 + (-10) = +1
This can be written as Na0 -----> Na+ + e-
Or like this Na0
Na+ + e-
|It only takes a relatively small amount of energy to take an electron away from the neutral sodium atom. So it is very easy to create a Na+ ion. To remove a second electron from Na+ an enormous amount of energy is required. This energy is not normally available and so sodium stops losing electrons as soon as it reaches a noble gas configuration.|
|Other metals act the same as well. Calcium usually forms
+2 charges. The amount of energy required to remove two electrons
is small. The amount needed to remove a third electron is far to high,
so calcium loses only two electrons and so achieves a noble gas
2s2 2p6 3s2 3p6
|It also sums to the correct amount. Ca+2 has 20 protons and 18 electrons which add up to a net charge of +2.|
|In the case of the metals, it is the stability of the noble gas core that lies beneath the outer shell of electrons that effectively limits the number of electrons that they can lose. The ions that form tend towards a noble gas configuration. The positive ions are collectively termed "cations".|
|A similar fate occurs to the non-metals which form
Negative ions are collectively termed "anions". Oxygen and
are typical nonmetals that form anions when they react with metals such
as calcium or sodium. When a chlorine atom reacts, it gains one
For a neutral chlorine atom we have
Cl0 1s2 2s2 2p6 3s2 3p5
|In this neutral atom there are 17 protons in the nucleus
electrons in orbit. (+17 + (-17) = 0) and when chlorine
it gains an electron Cl0 + e- -----> Cl-
|Cl-1 1s2 2s2 2p6 3s2 3p6|
|The full "p" subshell means that the chlorine ion now has
electronic configuration, is isoelectric, with Argon, a noble gas.
|At this point, the gaining of electrons stops, because if another electron were to be added, it would have to enter an orbital in the next higher shell.|
|With oxygen, a similar situtation exists. The formation of
ion, O2- gives oxygen a noble gas configuration isoelectric
with Ne. The gaining of two electrons happens very easily.
| O0 (1s2
2s2 2p4) + 2 e-
----> O2- (1s2 2s2
|Generalization: When a neutral atom form ions, atoms of most of the elements tend to gain or lose electrons until they have obtained a configuration that is the same as that of the nearest noble gas.|